1. Field of the Invention
The present invention relates to an encoding device and an encoding method that output an audio signal by multiplexing a first encoded data obtained by encoding a low-frequency component of the audio signal by a first encoding method and a second encoded data obtained by encoding a high-frequency component of the audio signal by a second encoding method. More particularly, the present invention relates to an encoding device and an encoding method that enable the high-frequency component of an audio signal to be appropriately encoded even when it is encoded in a low-resolution mode.
2. Description of the Related Art
Moving Picture Experts Group Phase 2 (MPEG-2) High-Efficiency Advanced Audio Coding (hereinafter, “HE-AAC”) method is a widely used method for encoding audio data such as voice and music. In the HE-AAC method, a low-frequency component of audio signals is encoded by AAC and a high-frequency component is encoded by Spectral Band Replication (SBR).
FIG. 8 is a schematic for explaining the HE-AAC method. Data encoded by the SBR method includes position data indicating the position where the high-frequency component is to be replicated from the low-frequency component (which is encoded by the AAC method), parameters representing correction of power of the high-frequency component, and data pertaining to components that cannot be replicated from the low-frequency component. As compared to other encoding methods, the data volume can be compressed to a much greater extent by encoding using the HE-AAC method, which combines the low-frequency component and the high-frequency component when encoding is performed by the AAC method. The data encoded by the AAC method shall hereafter be referred to as AAC data, and the data encoded by the SBR method shall be referred to as SBR data.
A conventional encoding device that encodes input audio data by the HE-AAC method is described below. FIG. 9 is a functional block diagram of the conventional encoding device. An encoding device 10 includes an SBR encoder 11, a down-sampling unit 12, an AAC encoder 13, and a multiplexing unit 14.
The SBR encoder 11 encodes input audio data by the SBR method, and outputs the encoded SBR data to the multiplexing unit 14. Prior to encoding the audio data, the SBR encoder 11 determines, based on criteria laid down beforehand by an administrator, whether the audio data is to be encoded in a high-resolution mode or a low-resolution mode and encodes the audio data according to the result of the determination.
FIG. 10 is a schematic for explaining the high-resolution mode and the low-resolution mode. The upper part of FIG. 10 is a schematic for explaining the high-resolution mode. In the high-resolution mode, the frequency bands of the input audio data being encoded by the SBR method (hereinafter, “SBR encoding band”) are divided into a plurality of blocks (for example, two blocks), and the power of each block is averaged out before the blocks are quantized and the SBR data created.
The lower part of FIG. 10 is a schematic for explaining the low-resolution mode. In the low-resolution mode, the power of the entire range of SBR encoded bands is averaged out and the block is quantized before SBR data is created. By encoding in the high-resolution mode, the high-frequency component of the audio data can be encoded accurately, and by encoding in the low-resolution mode, the data volume of high-frequency component can be reduced.
Returning to FIG. 9, the down-sampling unit 12 extracts the low-frequency component of the input audio data, and outputs the extracted low-frequency component to the AAC encoder 13. The AAC encoder 13 creates AAC data based on the low-frequency component received from the down-sampling unit 12, and outputs the AAC data to the multiplexing unit 14.
The multiplexing unit 14 multiplexes (combines) the SBR data output by the SBR encoder 11 and the AAC data output by the AAC encoder 13 and outputs the multiplexed data (HE-AAC bit stream). Thus, the conventional encoding device 10 encodes input audio data by the SBR encoder 11, the down-sampling unit 12, the AAC encoder 13, and the multiplexing unit 14.
A method is disclosed in Japanese Patent Application Laid-open No. 2005-338637 whereby the average power of every sub-band is compared before and after quantization, and if they are different, the scale factor (exponent) is adjusted so that the normalized power after quantization approximates the normalized power before quantization.
However, in the existing technologies, appropriate encoding of the high-frequency component is not realized when the high-frequency component of the input audio data is encoded in the low-resolution mode in order to reduce the data volume of the high-frequency components (the components of the input audio data in the SBR encoded bands).
The reason why the high-frequency component is not appropriately encoded is because, as shown in FIG. 10, if the entire high-frequency range is encoded in the low-resolution mode when the power at the high frequency end of the high-frequency component drops suddenly, the entire high-frequency component range is averaged, and the power at the high frequency end exceeds the power of the original audio data.
In other words, it is imperative to be able to appropriately encode the high-frequency component of the input audio data even when the high-frequency component is encoded in the low-resolution mode.